CMOS active pixel sensors are known, and have been described, for example, in U.S. Pat. No. 5,471,515. CMOS active pixel sensors are often very small, in order to make them compatible with current elements. While the device includes an in-pixel follower and/or other in-pixel transistors, size miniaturization often prevents a full complementary MOS type switch. This means, therefore, that a full-scale reset function within the active pixel circuit may be difficult.
In previous designs, this has prevented the operation from achieving full signal reset. This has led to smaller signal swing, lowered signal to noise ratio, and a lag effect in adjacent frames due to the incomplete reset. This kind of reset is often referred to as a soft reset. During such a soft reset, voltage levels on the sensor node may continuously change depending on the time that the reset persists. Ideally, this reset value should be set to the level Vdd−Vth, where Vdd is the drain supply and Vth is the threshold voltage of the transistor. However, the actual value may vary.
The soft reset may also limit the effective signal swing. This can be especially problematic in low voltage applications. For example, in a portable mode, the circuit may be powered from one or more batteries. Hence, the power supply may be limited to the battery voltage e.g. 1.5 V or 3 V levels.